THE INFLUENCE OF SEA-SURFACE TEMPERATURE ON THE TROPICAL INTRASEASONAL OSCILLATION - A NUMERICAL STUDY

The development and movement of the tropical intraseasonal system (TIS) exhibit remarkable annual variations. It was hypothesized that spatial and temporal variation in sea surface temperature (SST) is one of the primary climatic factors that are responsible for the annual variation of TISs. This paper examines possible influences of SST on the TIS through numerical experiments with a 2.5-layer atmospheric model on an equatorial beta plane, in which SST affects atmospheric heating via control of the horizontal distribution of moist static energy and the degree of convective instability. The gradient of the antisymmetric (with respect to the equator) component of SST causes a southward propagation of the model TIS toward northern Australia in boreal winter and a northward propagation over the Indian and western Pacific Oceans in boreal summer. The phase speed of the meridional propagation increases with the magnitude of antisymmetric SST gradients. The poleward propagation of the equatorial disturbance takes the form of moist antisymmetric Rossby modes and influences the summer monsoon. During May when SST is most symmetric in the western Pacific, a disturbance approaching the date line may evolve into westward-moving, double cyclonelike, symmetric Rossby modes due to the suppression of the moist Kelvin mode by the cold ocean surface east of the date line. The disturbance over the equatorial Indian Ocean, however, may evolve into an eastward-moving, moist Kelvin-Rossby wave packet; meanwhile, a cyclonic circulation may be induced over the Gulf of Thailand and Malaysia, drifting slowly westward into the Indian subcontinent.